Designing an Electrical Switchboard – 9 Things to Consider
A switchboard is the brains of your power network and houses everything to control the distribution of power and to keep you safe.
Below are 9 things you should think about when you are designing an electrical switchboard.
Does it conform to the Australian Standard for Main Switchboards
At the moment the Australian standard for main switchboards is AS/NZS 3439.1: 2002, which is based around IEC 60439. The new standard AS/NZS 61439 will supersede the current standard five years after its first publication. When this standard comes into effect, the customers in Australia can request the switchboard builders manufacture the boards in line with the new standard. If you need a copy of the standards you can get one here.
What is type testing?
Type testing basically means putting a standard design of switchboard through a series of electrical tests until it fails. Boards that are built to the same design are considered to comply with the type testing requirements and it is assumed the can withstand the same electrical forces on them. A switchboard made to this design is then classified as a ‘type tested assembly’ (TTA) in line with Australian Standard AS3439.1. This standard outlines the tests the switchboard must be put through.
What are the type tests that need to be carried out?
As part of a type testing these criteria must be verified:
- Temperature-rise limits
- Dielectric properties
- Short-circuit withstand strength
- The effectiveness of the protective circuit
- Clearances and creepage distances
- Mechanical operation
- Degree of protection and internal separation
What other tests have to be done with a switchboard that has type tested certificates in place?
All switchboards have to be tested to compliance by the manufacturer with four routine tests.
- Visual inspection of the switchboard – particularly the wiring, and potentially an electrical operation test.
- A Dielectric test must be carried out
- Insulation resistance testing
- Checking protective measures and testing the electrical continuity of the protective circuits.
What IP rating does it need to have?
Depending on where your switchboard is going to be located IP2X is the minimum for indoor use. Though for this there is no protection against the ingress of water.
For outdoor switchboards which are open to the elements IP23 is the minimum.
What is the minimum height from the ground/platform for the terminals of a circuit breaker?
It has to be at least 200mm according section 7.6 of AS/NZS 3439.1.
What is the maximum height?
For switchboards mounted on the floor, instruments which are to be read by a person should not be higher than 2 m above the base of the switchboard. This is the same for operating devices, such as handles and push-buttons.
What other environmental factors have to be taken into account for an outdoor switchboard?
If the switchboard is going to be installed in a place with high humidity and the temperature varying a great deal, then arrangements must be made for this. This might mean ventilation or internal heating prevent condensation building up inside the switchboard. There are separate standards for temporary construction site boards.
What are the temperature rise limits for equipment contained within a switchboard?
Temperature rise limits given in the standard or calculated apply for a mean ambient air temperature of <= 35°C. (So, this does not include outdoor switchboards where the ambient temperature goes above 35°C and where solar irradiation on the switchboard operating temperature is not known.)
For switchgear and control gear:
- Temperature rise should be considered as per the manufacturer’s recommendations.
- Generally switchgear is calibrated at a 35–40°C operating temperature. So a higher operating temperature (for instance in the Pilbara) would mean you have to derate the circuit breakers as per the manufacturer’s recommendations. You may also need to think about the maximum operating conditions to limit effect of temperature rise.
For Terminals for external conductors
- 70 kelvin rise
For Busbars and conductors
- Typically considered to be compliant providing the temperature rises do not go over 70 kelvin for H.C copper busbars and 55 kelvin for H.C aluminium busbars. This is based around 105°C and 90°C max temperature in an assembly.